Multiple-antenna technology is widely used in wireless communication systems such as cellular radio systems. For Long Term Evolution (LTE), Downlink (DL) Multiple Input Multiple Output (MIMO) was standardized by third generation partnership project (3 GPP) in release 8 (Rel 8 ). For Wideband Code Division Multiple Access (WCDMA) High Speed Packet Access (HSPA) evolution, downlink 2 by 2 MIMO was specified in Release 7 (Rel 7 ). The introduction of uplink multiple-antenna technology is being discussed in 3 GPP, including Uplink Transmit Diversity (ULTD) in WCDMA and uplink (UL) MIMO in LTE. With ULTD or UL MIMO, there will be at least 2 transmit (TX) antennas in the User Equipment (UE).
Known examples of ULTD include Switch Antenna Transmit Diversity (SATD) and Beam Forming Transmit Diversity (BFTD). For SATD, the UE transmits on the transmit (TX) antenna with better estimated uplink quality in terms of e.g. received Dedicated Physical Control Channel (DPCCH) power or DPCCH Signal to Interference plus Noise Ratio (SINR). For BFTD, the UE transmits on both TX antennas with an estimated weight factor to maximize received power or SINR. The ULTD technology can improve the system capacity and coverage, and save the UE battery consumption in most cases. There are two types of ULTD modes, Open Loop ULTD (OLTD) and Closed Loop ULTD (CLTD). In the latter case there is specific downlink feedback channel from the radio base station, Node B, to UE carrying the pre-coding vector or the information to assist the generation of pre-coding vectors of ULTD. For OLTD, there is no OLTD-specific downlink feedback channel from Node B to UE to support the ULTD operation. For CLTD, in order for the Node B to generate the desired pre-coding information to the UE over the specific downlink feedback channel, the Node B should also monitor the uplink channels. Traditionally the pre-coding vector is generated in the radio base station (Node B) side and sent to the UE. Alternatively the Node B can also send only the channel state information to UE, which in turn can autonomously create the pre-coding vector. Antenna imbalance is one key factor that impacts the system performance of OLTD, CLTD and UL MIMO.
OLTD for WCDMA-HSPA
For WCDMA-OLTD, the UE does the transmit adaptation of the 2 transmit antennas based on the available existing information. Algorithms for open loop SATD and for open loop BF are described in 3GPP Technical Report TR 25.863, “Uplink Tx Diversity for HSPA Study Item Technical Report”.
In case of SATD, there are two TX antennas and single full-power power amplifier in the UE. With the scheme described in TR 25.863, the UE selects the TX antenna according to the TPC statistic as follows:    1. Let TPC command DOWN be represented by −1 and TPC command UP by +1. Then let the UE accumulate all received TPC commands.    2. At each frame border the accumulated TPC sum is compared with 0. If the sum is larger than 0 the transmit antenna is switched.    3. If the same transmit antenna has been used for x consecutive frames the UE automatically switches antenna. x can be referred as the forced switch circle and determined according to the radio environments.    4. Every time an antenna switch occurs the accumulated TPC sum is reset to 0.
In case of BFTD, there are two power amplifiers in the UE side. With the algorithm described in 3GPP R1-102931, Concept of UL Closed Loop Transmit Diversity, the UE adjusts the beam by adjusting the phase bias between two antennas based on the received TPCs in accordance with the following:                A. The phase offset, δ, can be 48 degrees, ε can be 12 degrees.        B. Let TPC command DOWN be represented by −1 and TPC command UP by +1.                    1. Initial relative phase between two transmitters Δφ=−δ/2 for the first slot (#1 slot). ε is kept zero until two TPC commands become available to the UE.            2. Apply relative phase for the next slot Δφ=Δφ+δ            3. Determine new relative phase:                            a. if TPC1>TPC2, Δφ=Δφ+ε                b. if TPC2>TPC1, Δφ=Δφ−ε                c. otherwise, no change                                                Note that TPC1 and TPC2 correspond to slot (1,2 ),(3,4 ),. . . ,(i*2 −1 , i*2 ), where i=1 to n.                    4. Apply relative phase for the next slot Δφ=Δφ−δ            5. Go to step 2                        
A UL OLTD capable UE can be configured in default mode (fixed single TX antenna), OLTD mode (open loop SATD or BFTD).
CLTD in HSPA
CLTD was proposed in 3GPP RAN1-61 conference. The uplink closed loop transmit diversity scheme is based on the explicit uplink channel estimation and Channel Status Information (CSI) feedback was proposed. With this proposal, the network controls of the UE behavior and the adaptation of the transmit weights in a quickly. The simulation results show that the average throughput gain reaches 14% in Pedestrian A channel (3 km/h) and up to 10% in Vehicular A (30 km/h) channel. Overall CLTD can be a valuable complement that should be further considered for improving the HSPA uplink.
A UL CLTD UE can be configured in default TX antenna mode, open loop BFTD Mode and close loop BFTD mode.
UL MIMO in HSPA
For LTE, UL MIMO comprising of up to 4 transmit antennas in the UE is being specified, see RP-091430, UL multiple antenna transmission for LTE, work item, RAN#46. For WCDMA-HSPA, UL MIMO can be proposed naturally in 3GPP by companies after UL beam-forming is standardized since there is no hardware update for ULBF UE to support UL MIMO. In the current WCDMA-HSPA Node B, there are two receiver antennas. In the future, there can be even more receiver antennas and more advanced receiver technology that can be deployed in the network side, which means that there can be even larger gain from UL MIMO. For instance, the advanced receivers such as enhanced receiver Type 3 and interference cancellation technology can be deployed in the Node B side when the related cost is decreased to an acceptable level.
A UL MIMO capable UE can be configured in default single TX antenna mode, open loop BFTD Mode, close loop BFTD mode and UL MIMO mode.
There is a constant desire to improve upon existing transmission schemes. This is also true for configuration of uplink transmit diversity. Hence, there exist a need for an improved configuration of uplink transmit diversity